Agilent Technologies 34970A, Service Manual, Page: 138 / 273

A-to-D Converter
Unless otherwise noted, components in this discussion are located
on the A4 circuit assembly (34970-66504). The schematics are
included in Chapter 8 starting on page 234.
The analog-to-digital converter (ADC) is used to change dc voltages
into digital information (schematic shown on page 9-12). The circuitry
consists of an integrator amplifier (U402 and U420), current steering
switch U411, resistor network U102E, voltage reference U403, ADC
controller U209, and residue ADC U205.
The
ADC
method used is called multislope III . It is based on patented
Agilent ADC technology. Multislope III is a charge balancing
continuously integrating analog-to-digital converter. The ADC charge
balancing algorithm is always running, even when the multimeter is
not triggered. The input voltage continuously forces charge onto the
integrator capacitors C400 and C401 through U102E–R16.
Switches U411A and U411B steer fixed positive or negative reference
currents onto the integrator capacitor to cancel, or balance, the
accumulated input charge. The level shifted (R403 and R406) output
of the integrator is checked every 2.66
µ
s by the A1U209 COMP
input. Logic state machines in A1U209 control the U411 current
steering to continuously seek an approximate 2.5 V level on the
integrator amplifier output, FLASH. If the ADC input voltage ADIN
is between
±
15 V, the integrator output (FLASH) will remain within
the 0 V to 5 V range of the A1U205 on-chip ADC. An input greater
than +15 V may cause the integrator output (U402–6) to saturate at
about –18 V. An input less than –15 V may cause U402 to saturate
with an output of about +18 V. The A1U205 ADC input (FLASH) is
clamped to 0 V or 5 V by R405 and CR403 to protect A1U205.
The integrator amplifier is formed by U402 and U420. Resistors R420
and R421 affect the amplifier stability. Amplifier oscillation may
occur if their values are incorrect. Amplifier U420 improves the offset
voltage characteristics of integrator amplifier U402.
Chapter 5 Theory of Operation
Internal DMM
136